indexing_and_slicing_numpy_arrays.ipynb

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   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "# Indexing And Slicing Numpy Arrays\n",
      "\n",
      "- **Author:** [Chris Albon](http://www.chrisalbon.com/), [@ChrisAlbon](https://twitter.com/chrisalbon)\n",
      "- **Date:** -\n",
      "- **Repo:** [Python 3 code snippets for data science](https://github.com/chrisalbon/code_py)\n",
      "- **Note:**"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Slicing Arrays"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Explanation Of Broadcasting"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Unlike many other data types, slicing an array into a new variable means that any changes to that new variable are broadcasted to the original variable. Put other way, a slice is a hotlink to the original array variable, not a seperate and independent copy of it."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Import Modules"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import numpy as np"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 3
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Create an array of battle casualties from the first to the last battle"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "battleDeaths = np.array([1245, 2732, 3853, 4824, 5292, 6184, 7282, 81393, 932, 10834])"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 13
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Divide the array of battle deaths into start, middle, and end of the war"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "warStart = battleDeaths[0:3]; print('Death from battles at the start of war:', warStart)\n",
      "warMiddle = battleDeaths[3:7]; print('Death from battles at the middle of war:', warMiddle)\n",
      "warEnd = battleDeaths[7:10]; print('Death from battles at the end of war:', warEnd)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Death from battles at the start of war: [1245 2732 3853]\n",
        "Death from battles at the middle of war: [4824 5292 6184 7282]\n",
        "Death from battles at the end of war: [81393   932 10834]\n"
       ]
      }
     ],
     "prompt_number": 15
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Change the battle death numbers from the first battle"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "warStart[0] = 11101"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 20
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### View that change reflected in the warStart slice of the battleDeaths array"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "warStart"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 21,
       "text": [
        "array([11101,  2732,  3853])"
       ]
      }
     ],
     "prompt_number": 21
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### View that change reflected in (i.e. \"broadcasted to) the original battleDeaths array"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "battleDeaths"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 18,
       "text": [
        "array([11101,  2732,  3853,  4824,  5292,  6184,  7282, 81393,   932, 10834])"
       ]
      }
     ],
     "prompt_number": 18
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Indexing Arrays"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Create an array of regiment information\n",
      "\n",
      "Note: This multidimensional array behaves like a dataframe or matrix (i.e. columns and rows)"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "regimentNames = ['Nighthawks', 'Sky Warriors', 'Rough Riders', 'New Birds']\n",
      "regimentNumber = [1, 2, 3, 4]\n",
      "regimentSize = [1092, 2039, 3011, 4099]\n",
      "regimentCommander = ['Mitchell', 'Blackthorn', 'Baker', 'Miller']\n",
      "\n",
      "regiments = np.array([regimentNames, regimentNumber, regimentSize, regimentCommander])\n",
      "regiments"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 67,
       "text": [
        "array([['Nighthawks', 'Sky Warriors', 'Rough Riders', 'New Birds'],\n",
        "       ['1', '2', '3', '4'],\n",
        "       ['1092', '2039', '3011', '4099'],\n",
        "       ['Mitchell', 'Blackthorn', 'Baker', 'Miller']], \n",
        "      dtype='<U12')"
       ]
      }
     ],
     "prompt_number": 67
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### View the first column of the matrix"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "regiments[:,0]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 71,
       "text": [
        "array(['Nighthawks', '1', '1092', 'Mitchell'], \n",
        "      dtype='<U12')"
       ]
      }
     ],
     "prompt_number": 71
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### View the second row of the matrix"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "regiments[1,]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 72,
       "text": [
        "array(['1', '2', '3', '4'], \n",
        "      dtype='<U12')"
       ]
      }
     ],
     "prompt_number": 72
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### View the top-right quarter of the matrix"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "regiments[:2,2:]"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 75,
       "text": [
        "array([['Rough Riders', 'New Birds'],\n",
        "       ['3', '4']], \n",
        "      dtype='<U12')"
       ]
      }
     ],
     "prompt_number": 75
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [],
     "language": "python",
     "metadata": {},
     "outputs": []
    }
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   "metadata": {}
  }
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